i965/bufmgr: s/BO_ALLOC_FOR_RENDER/BO_ALLOC_BUSY/

i965/bufmgr: s/BO_ALLOC_FOR_RENDER/BO_ALLOC_BUSY/

"Alloc for render" is a terrible name for a flag because it means
basically nothing.  What the flag really does is allocate a busy BO
which someone theorized at one point in time would be more efficient if
you're planning to immediately render to it.  If the flag really means
"alloc a busy BO" we should just call it that.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>